Distributed sensors are an attractive and promising technique for monitoring parameters such as deformation and temperatures at long distances. Distributed fiber optic sensors are based on a modulation of the intensity or the frequency of the light introduced in the fiber and a synchronous detection so that we can determine the position in which the disturbance occurs. In general, any fracture or damage in the structure gives rise to a variation in the light intensity which is transmitted throughout the fiber. Within this type of sensors we can highlight sensors based on linear backscatter techniques and those based on non-linear effects such as Brillouin scattering and Raman scattering. In recent years, those based on Raman and Brillouin scattering have experienced a growing application in the instrumentation of all type of civil infrastructures (bridges, tunnels, buildings, dams), transport infrastructures (aeroplanes, railway lines, . . . ), industrial and energy infrastructures (gas and water pipelines, oil platforms, . . . ).
The Brillouin effect is a stimulated acoustic-optical interaction which is produced very efficiently in optical fibers. In simple terms, the Brillouin effect is obtained when laser light is introduced (spectrally narrow and sufficiently powerful) in an optical fiber. For the purposes of notation, we can assume that it is centred on an optical frequency f0. The presence of this light induces a gain on a light beam which is propagated in the opposite direction, to the frequency f0−νB. In this way, an attenuation is induced on a light beam which is propagated in opposite direction to the frequency f0+νB. The parameter νB is called Brillouin displacement and is sensitive to changes in temperature or deformation of the fiber. This fact is used for the distributed detection of changes in temperature and deformations.
Based on this physical phenomenon, different sensing techniques have been developed over the years, among which we can cite: BOTDA (Brillouin Optical Time Domain Analyzer); BOFDA (Brillouin Optical Frequency Domain Analyzer) and BOCDA (Brillouin Optical Correlation Domain Analysis); BOTDA or BOFDA with Raman assistance of any order and any configuration; V-BOTDA (Vectorial Brillouin Optical Time Domain Analyzer), wherein the probe and/or pump is modulated in some way; Coded-BOTDA; differential pump-width BOTDA (DPP_BOTDA); etc.; as well as any of these possible combinations.
In BOTDA sensors, and their variants, one or several pulses (in the case of coded-BOTDA, for example) and a continuous signal (modulated or not) counterpropagating to it are sent through the fiber to obtain the Brillouin frequency. To obtain the parameter νB, the frequency difference is analysed between the pulse sent and the continuous wave when the amplification of the counterpropagating continuous wave is maximized. To do this it is necessary to perform a frequency sweep. As the light is pulsed, the amplification recorded in the time domain shall also depend on the position at which the pulse is in at each instant. With this, it is possible to trace a Brillouin displacement map in accordance with distance. The Brillouin displacement variations can be related to temperature or deformation variations.
The case of the BOFDA sensors and their variants is differentiated from the above in that instead of a pulse an amplitude-modulated signal is sent through the fiber, with a variable frequency. To perform each measurement, a frequency sweep of the probe signal is necessary, in addition to a sweep in the modulation of the pump signal.
The sensors based on BOCDA technology and their possible variants make use of the fact that the stimulated Brillouin scattering depends on the correlation between the two waves which generate it and the efficiency of the process lowers sharply due to changes in frequency, phase or polarization. In short, the operation of the BOCDA is based in the artificial reduction of the correlation, by means of intelligent modulation, between the waves that generate stimulated Brillouin scattering at any point of the fiber, except at the point of study.
In the case of BOTDA and BOFDA-based sensors, there is an insurmountable and inherent limitation to the optical fiber which is the attenuation that the light undergoes on being propagated through it. The measurement length range that all these systems have is close to from a dozen meters to around fifty kilometers.
In the case of the BOCDA, the signals obtained are generally weak and it is convenient to have strategies to improve the SNR (Signal to Noise Ratio).
In all the systems mentioned above, the detection is performed at the end from which the pump is launched. For this, a photodetector is used to detect the probe signal in accordance with the flight time of the pump signal in the fiber. Normally, the probe signal is composed of two frequencies (one at f0+νB and another at f0−νB), with νB being the difference in frequency between pump and probe. The conventional systems isolate and/or filter in detection one of two frequencies which compose the wave and detect it with a single photodetector, obtaining gain or attenuation signals in accordance with the chosen band.
For all the distributed sensing systems based on stimulated Brillouin scattering described, there is in the state of the art the need to improve the dynamic range and the signal to noise ratio of the measurements, making it possible to achieve greater sensing lengths, maintaining the resolution.